The invention relates to a rolldynamometer or drumdynamometer for motor vehicles of the type specified in the preamble of the patent claim 1 as well as a method for controlling a plurality of electric driving motors of rolldynamometers.
For testing dynamic driving parameters as well as exhaust gas values of motor vehicles driven by internal combustion engines complex rolldynamometers or drumdynamometers enabling a simulation of various driving conditions have recently been used. For carrying out tests concerning, for example, the braking functions, the exhaust gas emissions or the likes, the vehicle to be examined is fixed in the rolldynanometer, for example, by means of a rod assembly, while either the two wheels of one axis or the two wheels of a plurality of axes are resting on the crown area of a drum or a pair of rolls, respectively, depending on the design of the dynamometer. The drums or pairs of rolls are coupled with driving/decelerating motors associated with measuring elements for detecting the torques or tensile forces occurring in correspondence with the different test conditions.
In dynamometers used for examining vehicles having a so called four wheel drive, the running rolls or drums for the front wheels and the running rolls or drums for the rear wheels must rotate with equal speeds or circumferential speeds even when the driving speed for the examination changes, since in case of possibly occurring speed differencences between the front and rear pairs of running rolls control actions of the electronics incorporated in the vehicle might be caused. For example, the braking systems of motor vehicles are normally designed so that the braking action applied to the front wheels is stronger than the braking action applied to the rear wheels when the brake pedal is operated. When examining the function of the braking system of the vehicle to be tested, the front wheels with the stronger brake action applied thereto may delay the running rolls of the front test set more, which may cause the anti blocking apparatus of the vehicle to assume an excessive slipping of the wheels and reduce the braking force of the front wheels correspondingly. When, on the contrary, the driven axis and with its wheels the corresponding pair of running rollers becomes faster than the non driven axis during an acceleration operation, this may cause a reaction of the anti slip controller of the vehicle which will then reduce the output of the internal combustion engine. It is plain to see that such control actions will falsify the measurement results.
In a dynamometer designed for function tests for different operation parameters of so called four-wheel-drive vehicles therefore not only the tractive resistances expressed by A, B and C coefficients and the forces and momentums of the moving masses resulting from an acceleration or deceleration have to be detected and controlled, but also the circumferential speeds of the front and rear running rolls or drums must be kept equal, an angularly synchronous rotation of the front and rear running rolls or drums being considered optimal.
From the U.S. Pat. No. 5,452,605 a drumdynamometer for a motor vehicle having a one-axis-drive is known in which a single test set comprises two running drums and a central electrical driving motor borne in a common support frame and driving both running drums directly and synchronously. The stator housing of the driving or deceleration motor is pendulously borne in two vertical posts and additionally supported on the frame via a torque or force sensor. When, for example, during a brake test on the motor vehicle, the braking force transmitted to the running rolls by its braked-down wheels is introduced into the driving motor, the stator housing reacts with a torsion within the limits determined by the torque sensor, the reaction moment of the stator housing being detected by the torque or force sensor. Changes of the torque applied by the stator housing are thus detected as measurement values by the torque sensor. For bearing both sides of the stator housing in the support frame, the stator housing is provided with a hollow tappet on each end face, the rotor shaft connected to the two running drums being borne by internal anti-friction bearings in each hollow tappet. Opposed to the support frame each hollow tappet is borne by another radially outer anti-friction bearing. A drawback of this bearing arrangement is that during a stillstand of the driving motor no lubrication film is present in the anti-friction bearings and that the bearing bodies contact each other directly. This results in extremely high friction values during the start-up of the motor and to a premature damage to the bearing arrangement. This adverse effect is increased by the risk that the roll bodies may sink into the bearing shell due to the structure-borne noise present during the operation of the motor.
For overcoming these adverse effects it is known from the U.S. Pat. No. 5,522,257 to coaxially mount two anti-friction bearings above each other with a central ring interposed between them, the central rings being driven with a predetermined rotational speed via a belt drive before the driving motor is turned on for generating a lubrication film in the two antifriction bearings before the driving motor is activated. There exists, however, a drawback in that a force is introduced duced by the movement of the central rings of these bearings, which force is added to the torque to be measured during the examination. Additionally, the double bearings and the central ring including its rotational drive increase the technical requirements.
From the DE-B-39 20 277 a rolldynamometer for four-wheel-drive motor vehicles is known by which so called yawing moments about the vertical axis of the vehicle may also be simulated and detected. Each front wheel and each rear wheel of the motor vehicle to be examined are supported by a pair of rolls, respectively. To individually drive and brake the wheels, each pair of rolls is driven by an own driving motor, a torque sensor being disposed between each roll and the associated driving motor. The driving motors are preceded by power converters which are each instructed by an associated gain control amplifier. For measuring the rotational speeds of the individual motors, tachometer generators are provided, one tachometer generator, one gain control amplifier, one power converter and one driving motor being the components of one closed loop speed control circuit and all control circuits being connected to a processor formed as a process control computer or programmable control unit. For simulating running through curves the different circumferential speeds of the drums on the inner curve side and the outer curve side corresponding to these curves can be determined with the aid of said processor based on a desired running speed.
It is the object of the invention to provide a rolldynamometer or drumdynamometer providing more accurate measurement results with a less complex construction.
According to the invention, this object is solved by the stator housing of the driving motor being borne in the support frame by bearing arrangements which are virtually friction free even during the activation or start-up operation.
According to a particularly efficacious embodiment of the invention the bearing arrangements are provided with plain bearings of simple design into the bearing parts (bearing shells) of which pocket shaped or groove like cavities are machined to which a largely constant carrying pressure oil flow is applied. Between the sliding surfaces of each bearing a film like cushion of pressure means of high support capacity and negligible friction values with respect to the relatively small pendulum motions of the motor housing in the support frame develops, if required. The torque applied by the motor housing during an examination operation is thus detected by the force meter without errors. One of said bearing arrangements is efficaciously formed as an axially fixed bearing and has an axial pressure bearing with integrated pockets in addition to the radial pressure bearing provided with the support pockets. Due to this so to say floating bearing of the stator housing the friction influences occurring in conventional anti-friction bearings as well as the wear of the bearing parts are avoided. While introducing radial forces due to, for example, the weight of a supported vehicle, the respectively inner bearing shell is pressed down whereby the gap widths of the lower half of these support bearings are reduced and thus the supporting property is increased.
Efficaciously one bearing shell of each plain bearing may be provided with an inner sliding layer, respectively, if required of PTFE, in which sliding layer grooves are formed in a net-like form. The pressure means supplied through these flat groves forms a thin film of large dimensions having a high support capability and minimum friction values.
For generating the oil flows, either a separate pump segment may be used for each support pocket, or a common pressure oil supply having only a single pump and individual control elements for the individual partial flows may be used as an alternative of simpler design. By designing volume flow controlling valves, a constant pressure oil flow may be set for achieving an approximately medium bearing hardness, for achieving a relatively hard bearing position a flow increasing with an increasing counter-pressure may be set, or for achieving a soft bearing position a flow reduced with an increasing counter-pressure may be set. With these possible variations a remarkably enhanced running smoothness is achieved as compared to anti-friction bearings, which may be decisive, for example, in noise examinations on the vehicle to be tested.
Another bearing arrangement for the xe2x80x9cpendulum bearingxe2x80x9d of the stator housing is characterised in that on both sides a plurality of angularly offset compound spring elements elastic in the circumferential direction are fixed to a collar attached to the support frame with their radially outer end and to a protruding component of each face wall of the stator housing with their radially inner end. Said compound spring elements are designed so that they offer only a negligible resistance to the relatively small turning movements of the stator housing and almost exclusively receive weight forces.
For further increasing the accuracy of the measurements, the drumdynamometer according to the invention is further characterised in that the force meter disposed between the stator housing and the frame construction is formed as a load cell one functional element of which is provided with a piston being floatingly supported in a cylinder into which a pressure oil flow is introduced on both sides. Due to this floating support of the functional member of the load cell the measuring errors unavoidable with conventional supports due to the friction in the multi-joint rod assembly as well as jams in the telescope rod assembly are avoided. In the embodiments conventionally formed as knuckle eyes and functioning as simple plain bearings oblique forces which may falsify the results of the measurements are introduced into the load cell due to static friction. The load cell used for the dynamometer according to the invention is, on the one hand, attached to the pendulously borne stator housing of the driving motor or the support frame and, on the other hand, connected to the piston accommodated in the flat cylinder fixed either to the support frame or to the stator housing. A pressure oil flow supplied either by a separate pump or by the pressure oil supply for the bearing arrangements and preferably having a constant volume is applied to the upper side and the bottom side of the piston, respectively. The control or metering of the pressure oil volume and the appropriate liquid pressure is effected by appropriate control elements securing a floating support of the piston and preventing impacts on the cylinder wall.
A drumdynamometer according to the invention is characterised in that a stationary first test set and a second test set movable in the longitudinal direction are disposed in a common pit so that the position of the two test sets may be adjusted to the different wheel bases of the vehicles to be examined for examining vehicles provided with a plurality of driven axes, each test set consisting of a support frame, a central electric driving motor with its stator housing borne pendulously in the support frame and two lateral drums of large volume being directly and synchronously driven or decelerated by the motor shaft protruding on both sides.
For a supportive cover of the spaces in the pit resulting from the movements of the second test set, slatted flexible and laterally stiff carrying straps forming the drive-on strips of variable length required for driving on and off the examination tools are advantageously fixed to the drive-on plates of said second test set in the longitudinal direction.
To keep the pit depth small even in case of larger running distances, another preferred embodiment of the invention is characterised in that the free end portions of these carrying straps preferably consisting of link chains with carrier plates are respectively accommodated in a magazine within the pit in which they are deviated in an U-shaped fashion.
A dynamometer for testing motor vehicles driven through one axis with their on-board electronics (ABS, ASC, etc.) deactivated is characterised by a single test set of the above described construction longitudinally movable within a relatively long pit by means of a motor and provided with the slatted covers.
To prevent increased emissions of the structure-borne noise inherent to the dynamometer, in a further embodiment of the invention, each test set may be supported on the pit bottom in an oscillation-isolated manner. In addition, the drive-on plates including a respective centring means for the vehicle wheel driven on may be formed as separate modules and fixed on the pit crown independently of the respective test set. Due to this positional and constructive separation between each test set and the drive-on components there arises the possibility to provide the upper structure with additional noise or heat isolations. Particularly during tests in environmental chambers the test sets and their driving motors as well as the corresponding measuring means will not have to be exposed to the same low temperatures as the vehicle to be examined due to such an insulation.
A further development of the invention particularly advantageous with respect to the complexity of the design and the safety of operation is characterised in that, according to the invention, synchronous motors parallelly operated using frequency converters are used as driving motors instead of the so far used DC or asynchronous motors. The output frequency fd of the converter is determined in accordance with the following formula:       f    d    =            ∑              xe2x80x83            ⁢              F        ·        t                    m      ·      U      ·      P      
wherein:
xcexa3F=sum of all forces (N)
t=time (s)
m=mass (kg) to be simulated
U=circumference of the respective running drum (m)
p=number of pole pairs of the synchronous motor.
On the basis of Newton""s second law the examination speed is given by the sum of the occurring forces divided by the simulated vehicle mass. The shaft speed is derived from the diameter of the drums, and a direct relation between the obtained converter frequency and the test speed is obtained when the pole pair number of the driving motors is taken into consideration. Probes for detecting rotary speeds and/or speeds are not required, which is an important advantage as compared to the conventional systems provided with a plurality of tachometer generators.
The excitation of the synchronous motors may be effected via slip rings or via a magnetic transformer. Using permanently excited motors is particularly advantageous. To suppress the pendulum oscillations known from synchronous motors as much as possible special damper rods may be included, for example, as cage windings. Due to the external excitation of the motors all motor shafts are operated in a frequency synchronised manner, the undesired pendulum oscillations of the respective driving motor being minimised by the cage windings provided in addition to the permanent excitation.
Another particularly efficacious system for obtaining a synchronous operation of the driving motors and their associated running drums is characterised in that the rotating masses are increased or decreased by simulating forces corresponding to the respectively occurring angular accelerations, said forces being added to or subtracted from the mass inertia. The corresponding electrically simulated additional masses are calculated in accordance with the following equation:       F    target    =            F      abc        +                  (                  m          ·                                    ⅆ              v                                      ⅆ              t                                      )            .      
Ftarget=the tensional force (N) to be applied by the driving motors
Fabc=tensional force of the driving resistances (N)
m=mass to be simulated
dv=speed change
dt=unit time
The advantage of this simulation is that any small simulated masses or no simulated masses may be set. A disadvantage of the method known from the DE-B-39 20 277 is that not any small mass may be simulated since a mass of xe2x80x9czeroxe2x80x9d would result in a division by zero in the calculation method applied here. Correspondingly, the rotating masses of the dynamometer must be kept relatively small in this known method so that the masses to be simulated for the vehicle must be correspondingly large. This, however, leads to the drawback that the driving motors simulate a relatively large portion of the vehicle mass and therefore have to be correspondingly large sized. For enabling the utilisation of relatively smaller and cheaper driving motors and to obtain an enhanced reaction with higher accuracy, the difference between the mechanically rotating masses and the translational vehicle masses to be simulated should be kept as small as possible.
Incidentally, a synchronous operation of the drums for the front wheels and the drums for the rear wheels is required while the function of the mass simulation is to be secured. In a preferred system according to the invention this is obtained by a detection of the angles of rotation of the front and the rear driving motor. By subtracting these two angles of rotation an angle difference is obtained which is supplied to a PID-controller. The output signal generated by said PID-controller is added to the actuation signal for the one driving motor and subtracted from the actuation signal for the second driving motor. Thus the sum of the torques remains the same for the mass simulation.
Instead of the otherwise common incremental encoder, so called sine/cosine encoders are advantageously used as rotary angle sensors in the system according to the invention. Said sine/cosine encoders are advantageous as compared to the digital encoders in that a correspondingly high resolution for the angle difference is obtained from the analogous voltage value of the sine/cosine signal. From the sine/cosine signal a normal quadrature signal usable, for example, for the vector control of the respective driving motor can be regenerated by a simple Schmitt-trigger.
Instead of the rotational angle of the two driving motors the circumferential speeds of the two running rolls may also be subtracted from each other. The speed difference is applied to a PID-controller, as described above, and the actuation signal thus generated is added to the actuation signal of the driving resistance controller or subtracted from the respectively other side. In contrast to the angle control described above this system offers only two constant test speeds (no angular synchronism of the drums), however, only simpler speed measuring elements are required.
Further, the distances run by the two drums may be subtracted from each other. The difference between the distances is applied to a PID-controller, as described above, and the thus generated signal is added to the actuation signal of the tractive resistance controller for the one driving motor and subtracted for the second driving motor.